A Novel Method Based on Hydrodynamic Cavitation for Improving Nitric Oxide Removal Performance of NaClO2

In the removal of nitric oxide (NO) by sodium chlorite (NaClO2), the NaClO2 concentration is usually increased, and an alkaline absorbent is added to improve the NO removal efficiency. However, this increases the cost of denitrification. This study is the first to use hydrodynamic cavitation (HC) combined with NaClO2 for wet denitrification. Under optimal experimental conditions, when 3.0 L of NaClO2 with a concentration of 1.00 mmol/L was used to treat NO (concentration: 1000 ppmv and flow rate: 1.0 L/min), 100% of nitrogen oxides (NOx) could be removed in 8.22 min. Furthermore, the NO removal efficiency remained at 100% over the next 6.92 min. Furthermore, the formation of ClO2 by NaClO2 is affected by pH. The initial NOx removal efficiency was 84.8–54.8% for initial pH = 4.00–7.00. The initial NOx removal efficiency increases as the initial pH decreases. When the initial pH was 3.50, the initial NOx removal efficiency reached 100% under the synergistic effect of HC. Therefore, this method enhances the oxidation capacity of NaClO2 through HC, realizes high-efficiency denitrification with low NaClO2 concentration (1.00 mmol/L), and has better practicability for the treatment of NOx from ships.


Introduction
Over 80% of global trade transport is through ships [1]. Furthermore, it is estimated that the average annual growth rate of international maritime transport trade will be 3.5% from 2019 to 2024 [2]. However, there are increasing concerns about the environmental problems caused by the ships' exhaust. The ships' exhaust mainly contains particulate matter (PM), nitrogen oxides (NO x ), carbon dioxide (CO 2 ), sulfur oxides (SO x ), and other substances hazardous to human health and the environment [3][4][5][6]. Additionally, NO x is the most difficult to remove [7,8]. Selective catalytic reduction (SCR) and exhaust gas recirculation (EGR) can remove NO from ship exhaust. SCR uses catalysts and ammonia to reduce NO x emissions. However, the removal of NO by SCR is greatly affected by temperature [9], and SO x and water in the exhaust gas from ships can cause catalyst poisoning [2]. EGR can reduce NO production at the source. However, increasing the EGR rate will lead to incomplete combustion, causing an increase in the PM [10] and reducing fuel economy [11]. SCR and EGR can only deal with NO, not SO 2 and PM simultaneously. Wet scrubbing technology has the advantage of treating multiple pollutants simultaneously and has attracted extensive attention from scholars [12]. The NO x emissions from ships consist of 90-95% NO. Moreover, NO is difficult to dissolve in water [13]. In wet removal of NO, it is oxidized with oxidants, and then NO x removal is promoted with absorbents [14]. High-potential oxidants include H 2 O 2 [15], Fenton-like reagents [16,17], persulfate salts [18,19], NaClO 2 [20], and KMnO 4 [21]. The absorbents include sodium humate (HA-Na) [22,23], NaSO 3 [24], and Ca(OH) 2 [25]. Hao et al. [26] compared the performance of different advanced oxidation

Reagents and Materials
The HC reactor was acquired from Mazzei Injector Company in Bakersfield, CA, USA; the Model 287 Venturi was used in the experiments. The cavitation chamber is constructed from glass-filled polypropylene. Structure and dimensions are depicted in detail in Figure S1 of the Supplementary Materials. This experiment also utilized a flue gas analyzer, a high-speed camera, a dryer, and a water purification system. The equipment used in the experiment is shown in Table 1. The reagents used in the experiment are shown in Table 2.

Experimental
As shown in Figure 1, the experimental setup was made of simulated gas, the HC reactor, the NaClO 2 solution, and the flue gas analyzer. The numbers 1 and 2 represent gas cylinders, and the numbers 3-8 represent valves. Different colors are used to depict the solution or gas in different states. The black, blue, red, green, and pink lines represent the simulated gas, NaClO 2 solution, gas-liquid mixture solution, treated exhaust, and reacted solution, respectively. Mass flow controllers regulated the flow rate of the simulated gas. The temperature of the NaClO 2 solution (3.0 L) was controlled by the thermostat bath. The differential pressure ∆P was regulated by valves 4 and 6.
In this study, the NaClO 2 solution was drawn from the thermostat bath through the pump. When the NaClO 2 solution flowed through the HC reactor at high speed, a lowpressure suction was created at the throat of the HC reactor, drawing the NO mixture. The gas-liquid mixture solution was separated by a gas-liquid separator. After being dried, the treated gas entered the flue gas analyzer for measurement. Simultaneously, the reacted solution flowed back into the thermostat bath through valve 7. In this study, the NaClO2 solution was drawn from the thermostat bath through the pump. When the NaClO2 solution flowed through the HC reactor at high speed, a lowpressure suction was created at the throat of the HC reactor, drawing the NO mixture. The gas-liquid mixture solution was separated by a gas-liquid separator. After being dried, the treated gas entered the flue gas analyzer for measurement. Simultaneously, the reacted solution flowed back into the thermostat bath through valve 7.

Nomenclature and Calculation of Removal Efficiency
The nomenclature and notation used in this study are shown in Table 3. NOx concentration in the treated gas is calculated as follows: C NO x , out = C NO, out + C NO 2 , out The removal efficiencies of NO and NOx can be calculated by the following equations:

Nomenclature and Calculation of Removal Efficiency
The nomenclature and notation used in this study are shown in Table 3. The initial removal efficiency of NO x with an initial pH of 4 − 7 (%) The maximum removal efficiency of NO x with an initial pH of 4 − 7 (%) The time of η NO = 100% (min) C v Cavitation number NO x concentration in the treated gas is calculated as follows: The removal efficiencies of NO and NO x can be calculated by the following equations: where C in is the concentration of NO in simulated gas. C NO, out and C NO x , out are the concentrations of NO and NO x , respectively.

Measurement of Gas Concentration and pH
First, high-purity N 2 is used to clean the oxygen (O 2 ) in the experiment. The experiment started when the O 2 content dropped to 0.00%. Next, the pH meter and flue gas analyzer record the data regularly, with the counting interval uniformly set to 5 s. When valve 5 is opened, the simulated NO gas is introduced into the system, and data recording is initiated. Cavitation is the generation, growth, and collapse of cavities when the local pressure in the liquid is lesser than the saturated vapor pressure at the local temperature. As illustrated in Figure 2a, the NaClO 2 solution moving at high speed enters the HC reactor from A and forms a low suction pressure. The NaClO 2 solution with dissolved NO mixture forms local cavities at low pressure. According to Gogate's research, the pressure at the moment of cavitation is generally lower than the saturated vapor pressure at the corresponding temperature [40]. Under low pressure, the cavitation liquid film tends to evaporate inward, thereby balancing the pressure difference between the interior and exterior of the cavities. As the pressure decreases further, the bubble expands rapidly. The cavity is continuously filled with molecules of gas evaporated from the liquid film. At X 2 -X 3 in Figure 2a, when the NaClO 2 solution flows through the throat of the HC reactor, the flow area becomes more significant, and the pressure on the NaClO 2 solution can recover rapidly. As shown in Figure 2b, the volume of cavities decreases continuously under the restoring pressure. Since the compression process of the cavity is extremely short, it can be considered an adiabatic compression process. Rapid compression raises the temperature of the cavities sharply. It forms hot spots with high local temperature and pressure of 5000-15,000 K [41][42][43][44][45] and 100-500 MPa [46][47][48], respectively, ultimately leading the cavity to collapse. As shown in Figure 2c, the collapse of the cavity results in the formation of many tiny bubbles and microjets. The cavitation process promotes chemical reactions through mechanical, thermal, and chemical effects, strengthening NO x removal.

Results and Discussion
As shown in Figure 2d, under the cavitation conditions, • OH and • H radicals are produced by the pyrolysis of water molecules [49] (as given by Equation (4)).
The • OH radicals have a strong oxidation capacity with a redox potential of 2.80 eV [50]. Additionally, NO or NO 2 may react with • OH either inside or on the surface of the cavities, finally oxidizing to nitric acids (HNO 3 ) and nitrous (HNO 2 ) [34] (as given by Equations (5)- (7)).
Additionally, • H radicals have an extremely strong reduction capacity and may react with NO or NO 2 (as given by Equations (8) and (9)).
In a previous study, the size of bubbles produced at the outlet of the HC and bubbling reactors were compared. It was found that the size of the bubbles in the HC reactor (0.62 mm) was far smaller than those in the bubbling reactor (23.19 mm) [34]. As shown in Figure 2a, the low suction pressure is generated at the throat of the HC reactor. the HC reactor creates low suction pressure in the throat, drawing NO from B. Consequently, the flowing NO was cut by the NaClO 2 solution flowing at high speed and forming many gas-filled bubbles. The gas-filled bubbles are formed at low pressure, and when the HC reactor's restoring pressure compresses them, their volumes become smaller (0.50-1.50 mm).
Furthermore, since the gas-filled bubbles are slowly compressed, they cannot collapse. However, the small space inside them increases the collision between NO and • OH or ClO 2 , which is conducive to the gas-phase chemical reaction. Additionally, compression of the gas-filled bubbles increases their temperature. Consequently, it increases the impact speed and frequency of NO molecules on the surface of the gas-filled bubbles, enhancing the gas-liquid mass transfer [51,52]. As shown in Figure 2d, under the cavitation conditions, •OH and •H radicals are produced by the pyrolysis of water molecules [49] (as given by Equation (4)).
The •OH radicals have a strong oxidation capacity with a redox potential of 2.80 eV [50]. Additionally, NO or NO2 may react with •OH either inside or on the surface of the cavities, finally oxidizing to nitric acids (HNO3) and nitrous (HNO2) [34] (as given by Equations (5)- (7)).

Effect of ∆P on NO Removal
Furthermore, the differential pressure ∆P was adjusted to promote the occurrence of cavitation. The cavitation number C v decreases when ∆P increases. As C v decreases, the cavitation intensity increases. Meanwhile, more reactive radicals may be generated by a higher cavitation intensity which is conducive to NO x removal. Additionally, C v is defined as follows: where, P 3 , P, V, and ρ denote the outlet pressure of the HC reactor, the vapor pressure of the liquid at saturation temperatures, the velocity of the liquid at the HC reactor throat, and the liquid density, respectively. Furthermore, ideally, cavitation occurs at C v ≤ 1. However, since the introduction of the NO mixture in this study causes the solution to contain dissolved gas, cavitation occurs at C v > 1 [17,53]. A transparent acrylic tube (Length: 300.00 mm, Outside diameter: 30.00 mm) was connected to the HC reactor to observe the gas-filled bubbles at the outlet, as shown in Figure 3a. Then a high-speed camera was used to capture the gas-filled bubbles in the 40.00 mm area of the acrylic tube. As illustrated in Figure 3b, as ∆P increases, the size of the gas-filled bubbles at the outlet decreases, and they are gathered more densely. As shown in Figure 3c, the diameters of the gas-filled bubbles were about 0.62 mm with ∆P = 3.00 bar, while they were 0.53 mm with ∆P = 5.00 bar. The surface area and the volume of gas-filled bubbles with ∆P = 5.00 bar were 4.60 and 0.36 times the amount of their equivalents with 3.00 bar. A higher ∆P promotes mixing gas and liquid to increase the contact area. Therefore, the chemical reaction rate accelerates with increasing ∆P for a certain time. A higher ∆P also increases liquid velocity, reducing the overall chemical reaction time. As shown in Figure 3c, when ∆P increased from 3.00 bar to 6.00 bar, the velocity of the gas-filled bubbles increased by 0.20 m/s, and the contact time between gas and liquid reduced by 0.20 s.
Furthermore, the increase in ∆P leads to an increased rate of chemical reaction and shortened reaction time, and this competitive effect affects the duration of the NO x removal efficiency, η NOx . As illustrated in Figure 3d, as ∆P increases, the time of η NOx = 100%, Tη NOx,100% , first increases and then decreases. When ∆P was 3.00 bar, Tη NOx,100% was 3.92 min. Tη NOx,100% was maximum (8.22 min) and minimum (1.92 min) when ∆P was 5.00 bar and 6.00 bar, respectively. Therefore, the competitive effect was balanced when ∆P was 5.00 bar.
Furthermore, when η NOx is in the range of η • −99.9%, only NO 2 is detected in the treated exhaust. Additionally, the oxidation capacity of the NaClO 2 solution still keeps 100% NO removal efficiency. The highest NO 2 concentration in the treated gas is reached when η NOx is η • . When ∆P was 3.00 bar, 4.00 bar, 5.00 bar, and 6.00 bar, η • was equal to 89.6%, 86.9%, 86.5%, and 82.1%, and the maximum NO 2 concentration was equal to 104 ppmv, 131 ppmv, 135 ppmv, and 179 ppmv, as shown in Figure 3d,f, respectively. The maximum NO 2 concentration at ∆P = 5.00 bar was 135 ppmv, which is higher than that at ∆P = 3.00 bar. When the NO 2 concentration reaches the maximum, η NO decreases from 100%. At that moment, the NaClO 2 solution cannot oxidize NO completely. At ∆P = 5.00 bar, Tη NO,100% (15.14 min) was 5.58 min longer than Tη NO,100% (9.42 min) at ∆P = 3.00 bar, as shown in Figure 3c. When the NO 2 concentration reaches the maximum, the NaClO 2 consumption at ∆P = 5.00 bar was larger than that at ∆P = 3.00 bar. Therefore, the maximum concentration of NO 2 increased. When ∆P = 6.00 bar, Tη NOx,100% was only 1.92 min, and the maximum concentration of NO 2 (179 ppmv) was reached at the 16th min. Therefore, there was a significant increase in the maximum NO 2 concentration.
velocity of the gas-filled bubbles increased by 0.20 m/s, and the contact time between gas and liquid reduced by 0.20 s.  As illustrated in Figure 3e, Tη NO,100% was the longest for ∆P = 6.00 bar. The primary reason for the increase in the NO 2 concentration was that large amounts of ClO 2 escape due to a high ∆P. The high ∆P results in lower suction and pressure of the gas-filled bubbles, which is more conducive for vaporizing the liquid into the bubbles. NaClO 2 generates adequate ClO 2 rapidly for initial pH of 3.50 (as given in Equation (11)). Therefore, at the high ∆P, ClO 2 in the liquid phase is more likely to be vaporized into and discharged together with the gas-filled bubbles.
Additionally, a high ∆P shortens the reaction time. Furthermore, the absorption of NO 2 becomes insufficient due to the short contact time between gas and liquid. NO 2 requires time to be converted to N 2 O 3 and N 2 O 4 (as given in Equations (12) and (13)), which were dissolved by the liquid phase (as given in Equations (14) and (15)) [20,54,55]. It is generally accepted that the increase in nitrogen valency increases the solubility of gaseous nitrogen in the aqueous phase [56]. Therefore, the short reaction time would inhibit this process, and the absorption of NO 2 would become insufficient.
Summing up, for ∆P = 5.00 bar, the influence of increased reaction rate and shortened reaction time reached a good balance. Furthermore, the maximum NO 2 concentration was only 135 ppmv, and Tη NOx,100% was the maximum (8.22 min). Therefore, ∆P = 5.00 bar was used as the experimental optimal ∆P.

Effect of Initial pH of NaClO 2 Solution on NO Removal
According to the Nernst equation, the reduction potential of NaClO 2 decreases as the pH increases. However, since pH affects the generation of NaClO 2 to ClO 2 , there is an optimal pH for NO x removal [13,57,58]. Yang et al. [59] and Adewuyi et al. [58] suggested removing NO x by NaClO 2 in neutral or slightly acidic conditions. Therefore, the experiments were first performed at an initial pH of 4.00−7.00 in this study. As the initial pH decreases, the η NOx initial increases. As illustrated in Figure 4a,b, for the initial pH range of the solution of 4.00−7.00, the initial NO x removal efficiency η NOx initial is 84.8−54.8%. Since, at this time, the amount of ClO 2 generated by NaClO 2 was not enough to oxidize NO x completely, the η NOx initial could not reach 100%. Furthermore, as the solution absorbs more NO x , its pH gradually decreases, and η NOx reaches its maximum value η NOx max . When the initial pH was 4.00, 5.00, 6.00, and 7.00, the values of η NOx max were 87.4%, 71.9%, 65.4%, and 64.4%. The increase in η NOx max is by 2.6%, 11.9%, 8.5%, and 9.6%, respectively, compared to η NOx initial . As shown in Figure 4c, the instantaneous pH range of achieving η NOx max is 3.50−3.70, according to the experimental results. The authors of this study were of the opinion that adjusting the initial pH to 3.50−3.70 may improve η NOx initial , so the experiment was carried out for initial pH = 3.50. Subsequently, it was shown that η NOx initial could reach 100% for the initial pH = 3.50. When the initial pH of the NaClO 2 solution is 4.00 − 7.00, the NO x of treated emissions consists of NO and NO 2 . NO was not completely oxidized, and NO 2 was not completely absorbed, resulting in the η NOx initial being less than 100%. The reduction of initial pH could significantly improve the oxidation capacity of the NaClO 2 solution. Gong et al. [60] explained that the NO removal efficiency increased with the decrease of pH. A 100% removal efficiency of NO could be achieved when the pH was below 2.5. In addition, when the initial pH of the NaClO 2 solution is 4.00−7.00, due to the reduced amount of ClO 2 generated, NO 2 was not completely absorbed. Song et al. [37] carried out research on the removal of NO 2 by H 2 O 2 , NaS 2 O 8 , NaClO 2 , and ClO 2 under HC conditions, and reported that ClO 2 has a higher oxidation selectivity for NO 2 compared with NaClO 2 . When the initial pH was 4.00−7.00, the ClO 2 generated per unit of time was small [61]. When the experiment was carried out for initial pH = 3.50, the amount of ClO 2 generated per unit of time was more significant [62]. Therefore, η NOx initial could reach 100% for the initial pH = 3.50.  Furthermore, the reduction of initial pH could significantly improve the NOx removal efficiency. Experiments with an initial pH of 2.00−3.50 were carried out in this study to explore further the influence of initial pH on removing NO based on HC combined Furthermore, the reduction of initial pH could significantly improve the NO x removal efficiency. Experiments with an initial pH of 2.00−3.50 were carried out in this study to explore further the influence of initial pH on removing NO based on HC combined with NaClO 2 , so an acidic oxidation mechanism was followed between NO and NaClO 2 [63]. Therefore, NO is removed by reacting with ClO 2 − (as given by Equation (16)) or ClO 2 (as given by Equation (17)). In addition, • OH (as given by Equations (5) and (6)) and • H (as given by Equation (8)) will also promote the removal of NO. As shown in Figure 4d, when the initial pH was 2.00, 2.50, 3.00, and 3.50, η NOx initial reached 100% and maintained this value for more than 8 min. Furthermore, this depends on the rapid decomposition of NaClO 2 to generate ClO 2 in acidic conditions (as given in Equation (11)). A significantly low value of pH shortens Tη NOx,100% . When the initial pH was 2.00, 2.50, and 3.50, Tη NOx,100% was 8.50 min, 9.22 min, and 9.43 min, respectively, as shown in Figure 4d. The reason for this phenomenon may be the escape of excess ClO 2 from the liquid phase [64]. The reaction rate of Equation (11) may be influenced by the ClO 2 − and H + concentrations. The reaction rate of Equation (11) is faster for higher ClO 2 − and H + concentrations, and more ClO 2 is produced per unit of time. Tη NOx,100% was the maximum for initial pH = 3.00, and the production of ClO 2 was sufficient for NO removal (1000 ppmv, 1.0 L/min) in a unit of time. However, ClO 2 was overproduced for initial pH of 2.00 and 2.50. The excessive ClO 2 vaporized into gas-filled bubbles and discharged together with them. Thus, NaClO 2 consumption was accelerated, leading to a reduction of Tη NOx,100% .
When η NOx was in the interval of η • −99.9%, the NaClO 2 solution concentration decreased, and the reaction rate of Equation (11) became slow. In this case, the influence of ClO 2 escaped on the duration of the interval of η NOx became smaller, and the remaining NaClO 2 in the solution had a more significant influence on the duration of the interval. The amount of NaClO 2 remaining in the solution became lesser as the duration of the interval lengthened. When the initial pH was 2.00, 2.50, 3.00, and 3.50, the duration of the interval of η NOx was 4.21 min, 4.37 min, 5.64 min, and 6.92 min, respectively, as shown in Figure 4d. Simultaneously, the remaining NaClO 2 in the solution also affected the maximum NO 2 concentration in the treated gas. As shown in Figure 4f, the maximum NO 2 concentration generally declines. When the initial pH was 2.00, 2.50, 3.00, and 3.50, the maximum NO 2 concentration was 205 ppmv, 182 ppmv, 195 ppmv, and 135 ppmv, respectively. Chin et al. [65] and Brogren et al. [66] explained that 60−80% of the NO 2 generated in the reaction can be removed by Equations (18)- (22).
• ClO + NO 2 → ClONO 2 (24) ClONO 2 +H 2 O → HOCl + HNO 3 (25) When the initial pH was 3.50, 100% NO x removal efficiency was maintained for 8.22 min. Subsequently, NO 2 was detected in the treated gas, but the oxidation capacity of the NaClO 2 solution could still maintain 100% NO removal efficiency for 6.92 min. η NOx decreased from η • to 0.0% (η • = 86.5%), the NO concentration increased from 0 ppmv to 1000 ppmv, and NO 2 concentration rapidly decreased from 135 ppmv to 0 ppmv in the next 3.67 min. Therefore, this indicated that the decreased NaClO 2 solution concentration led to the loss of oxidation capacity for NO removal. Therefore, for the NO x removal by NaClO 2 solution under acidic conditions, the fundament was the rapid activation of ClO 2 , and the increasing Tη NOx,100% required improved NO 2 absorption. A lower value of initial pH increased Tη NOx,100% , but a large amount of escaping ClO 2 led to a reduced oxidation capacity and a higher NO 2 concentration in the solution. The lower pH can also be a severe concern for the corrosion of the experimental equipment. Therefore, the optimal initial pH of the solution was taken as 3.50 in this study.

Effect of Reaction Temperature on NO Removal
The reaction temperature significantly influences the dissolution and diffusion of molecules or ions in the NaClO 2 solution. Additionally, the change in the reaction temperature would affect the change in the saturated vapor pressure of the solution, affecting the cavitation. According to the Arrhenius law, a high temperature promotes ion diffusion and accelerates chemical reactions [10]. The high temperature promotes the thermal decomposition of NaClO 2 to generate ClO 2 (as given in Equation (11)) [67]. As the reaction temperature increased, Tη NOx,100% first increased and then decreased. When the reaction temperature was 30.  Figure 5a. Therefore, the increase in temperature had the same influence on Tη NO,100% . As shown in Figure 5b, Tη NO,100% was the shortest (10.60 min) for 30.0 • C reaction temperature. Additionally, Tη NO,100% was the longest (16.83 min) for 50.0 • C reaction temperature. However, Tη NO,100% decreased to 14.50 min for 60.0 • C reaction temperature. The decrease in the Tη NO,100% value indicated that NO could not be fully oxidized. This was because the high temperature accelerated the thermal decomposition of NaClO 2 into ClO 2 , which led to the consumption of NaClO 2 in the solution.
When the initial pH was 3.50, 100% NOx removal efficiency was maintained for 8.22 min. Subsequently, NO2 was detected in the treated gas, but the oxidation capacity of the NaClO2 solution could still maintain 100% NO removal efficiency for 6.92 min. ηNOx decreased from η• to 0.0% (η• = 86.5%), the NO concentration increased from 0 ppmv to 1000 ppmv, and NO2 concentration rapidly decreased from 135 ppmv to 0 ppmv in the next 3.67 min. Therefore, this indicated that the decreased NaClO2 solution concentration led to the loss of oxidation capacity for NO removal. Therefore, for the NOx removal by NaClO2 solution under acidic conditions, the fundament was the rapid activation of ClO2, and the increasing TηNOx,100% required improved NO2 absorption. A lower value of initial pH increased TηNOx,100%, but a large amount of escaping ClO2 led to a reduced oxidation capacity and a higher NO2 concentration in the solution. The lower pH can also be a severe concern for the corrosion of the experimental equipment. Therefore, the optimal initial pH of the solution was taken as 3.50 in this study.

Effect of Reaction Temperature on NO Removal
The reaction temperature significantly influences the dissolution and diffusion of molecules or ions in the NaClO2 solution. Additionally, the change in the reaction temperature would affect the change in the saturated vapor pressure of the solution, affecting the cavitation. According to the Arrhenius law, a high temperature promotes ion diffusion and accelerates chemical reactions [10]. The high temperature promotes the thermal decomposition of NaClO2 to generate ClO2 (as given in Equation (11)) [67]. As the reaction temperature increased, TηNOx,100% first increased and then decreased.  Figure 5a. Therefore, the increase in temperature had the same influence on TηNO,100%. As shown in Figure 5b, TηNO,100% was the shortest (10.60 min) for 30.0 °C reaction temperature. Additionally, TηNO,100% was the longest (16.83 min) for 50.0 °C reaction temperature. However, TηNO,100% decreased to 14.50 min for 60.0 °C reaction temperature. The decrease in the TηNO,100% value indicated that NO could not be fully oxidized. This was because the high temperature accelerated the thermal decomposition of NaClO2 into ClO2, which led to the consumption of NaClO2 in the solution.  However, the high temperature decreases the solubility of NO x or ClO 2 . Additionally, it enhances the mass-transfer resistance between gas and liquid, resulting in a decreased mass transfer of NO from the gas to the liquid phase. Therefore, when the reaction temperature exceeded 50.0 • C, NO x absorption was inhibited due to the decrease in NO x solubility. Therefore, as temperature increased, Tη NO,100% first increased and then decreased. An increase in temperature would also increase the maximum NO 2 concentration in the treated gas. Furthermore, when the temperature increased from 30.0 • C to 60.0 • C, the maximum NO 2 concentration increased from 84 ppmv to 175 ppmv, as shown in Figure 5c. Nitrites of the solution decomposed into NO 2 at higher reaction temperatures (as given in Equation (26)) [67], which may be one of the reasons for an increase in the maximum NO 2 concentration with the increase in temperature.
In addition to this, a temperature change will cause a change in cavitation intensity. The influence of temperature on cavitation intensity is mainly through viscous and thermodynamic effects [32]. As temperature increases and viscosity decreases, the Reynolds number increases proportionally. The generation of turbulence effects increases the intensity of cavitation. Temperature increases the vapor pressure, making it easier for the NaClO 2 solution to evaporate and accelerating the formation of cavities [68]. Therefore, the values of Tη NOx,100% and Tη NO,100% keep increasing as the temperature rises from 30 • C to 50 • C. However, too high of a temperature will have a delay effect on cavitation. Brennen quantifies the delays of cavitation with the thermodynamic parameter Σ [69], as follows: where T ∞ is the test temperature, ρ V is the vapor density, ρ l is the liquid density, L is the evaporative latent heat, c p,l is the constant pressure specific heat of the liquid, and α l is the thermal diffusivity of the liquid. The ∑ parameter depends only upon the liquid's temperature; thus, various liquids can be compared to each other regarding the thermal delay. Hattori et al. [70] reported that the thermodynamic effect becomes significant when the thermodynamic parameter ∑ = 100 m/s 3/2 . For water, the applicable range is 50 • C and 55 • C. When the temperature in this study exceeds 50 • C, the thermodynamic effect significantly retards the development of cavitation. At this point, the increase in vapor pressure tends to evaporate the liquid, causing cavities to merge and reducing the number of individual cavitation structures [68]. The delay in cavitation causes a reduction in cavitation intensity. Therefore, the chemical effect of cavitation will also be weakened, and the production of • OH and • H (as given in Equation (4)) and • ClO (as given in Equation (23)) will be reduced, which is not conducive to the removal of NO x (as given in Equations (5)- (9) and (24) and (25)).

Effect of NaClO 2 Concentration on NO Removal
The increased concentration of NaClO 2 enhanced the mass transfer effect between the gas and liquid phases. As illustrated in Figure 6a, when the NaClO 2 concentration was 0.60 mmol/L, η NOx reached 100%. However, the duration was only 0.50 min. A short duration is not conducive to observing the complete reactive trend of NO x removal by HC. Furthermore, when the concentration of NaClO 2 increased from 0.60 mmol/L to 1.40 mmol/L, Tη NOx,100% increased from 0.50 min to 11.50 min. Therefore, Tη NOx,100% is linear with the NaClO 2 concentration (as given in Equation (28) Furthermore, when the NaClO2 concentration was 1.00 mmol/L, TηNOx,100% was 8.22 min, which was 1.84 min higher than the predicted value of 6.38 min in Equation (28). As the concentration of NaClO2 increases, the amount of ClO2 produced will also increase. However, when the NaClO2 concentrations were 0.60 mmol/L, 0.80 mmol/L, 1.20 mmol/L, and 1.40 mmol/L, the corresponding TηNOx,100% values were lower than the predicted values, as shown in Figure 6.
In addition to this, as illustrated in Figure 6b, when the concentrations of NaClO2 were 1.20 mmol/L, and 1.40 mmol/L, TηNO,100% values were 15.25 min and 16.25 min, respectively. Furthermore, compared with the TηNO,100% value of 15.14 min for NaClO2 concentration of 1.00 mmol/L, they showed an increase of 0.11 min and 1.11 min, respectively. However, when the concentration exceeded 1.00 mmol/L, TηNO,100% values did not increase significantly. Additionally, the increase in NaClO2 concentration did not significantly reduce the average concentration of NO2 (135 ± 13 ppmv), as shown in Figure 6c. The maximum concentration of NO2 was 137 ppmv when the NaClO2 concentration was 1.20 mmol/L, which was 2 ppmv higher than when the NaClO2 concentration was 1.00 mmol/L. NO2 was transported by the escape of ClO2, resulting in a higher concentration of NO2 [35]. Therefore, the optimal NaClO2 concentration was considered as 1.00 mmol/L in this study.

Conclusions
Detailed experiments were carried out to study the influence of various parameters on NO removal efficiency, including the ∆P of the HC reactor, the initial pH, the reaction temperature, and the concentration of NaClO2. The experimental results showed that removing NO from ship exhaust based on HC using NaClO2 solution was a valid method. The advantages of this novel method were low NaClO2 concentration and high NOx removal efficiency. The NOx removal efficiency reached 100% for the NaClO2 concentration of 0.60 mmol/L. The HC reactor could generate many gas-filled bubbles with small volumes, which was conducive to enhancing the contact area between liquid and gas to accelerate the reaction rate. The reduction of initial pH could significantly improve the oxidation capacity of the NaClO2 solution. ηNOx initial was below 100% for the initial pH = 4.00-7.00. When the initial pH ≤ 3.50, ηNOx initial reached 100% and was maintained for more than 8 min. The fundamentals for NOx removal by NaClO2 solution under acidic conditions was the rapid activation of ClO2, and the increasing TηNOx,100% required improved NO2 absorption. Additionally, • OH and • ClO produced by HC promoted the NO2 absorption, which may be one of the reasons for complete NO2 removal when ηNOx was 100%. Furthermore, when the NaClO 2 concentration was 1.00 mmol/L, Tη NOx,100% was 8.22 min, which was 1.84 min higher than the predicted value of 6.38 min in Equation (28). As the concentration of NaClO 2 increases, the amount of ClO 2 produced will also increase. However, when the NaClO 2 concentrations were 0.60 mmol/L, 0.80 mmol/L, 1.20 mmol/L, and 1.40 mmol/L, the corresponding Tη NOx,100% values were lower than the predicted values, as shown in Figure 6.
In addition to this, as illustrated in Figure 6b, when the concentrations of NaClO 2 were 1.20 mmol/L, and 1.40 mmol/L, Tη NO,100% values were 15.25 min and 16.25 min, respectively. Furthermore, compared with the Tη NO,100% value of 15.14 min for NaClO 2 concentration of 1.00 mmol/L, they showed an increase of 0.11 min and 1.11 min, respectively. However, when the concentration exceeded 1.00 mmol/L, Tη NO,100% values did not increase significantly. Additionally, the increase in NaClO 2 concentration did not significantly reduce the average concentration of NO 2 (135 ± 13 ppmv), as shown in Figure 6c. The maximum concentration of NO 2 was 137 ppmv when the NaClO 2 concentration was 1.20 mmol/L, which was 2 ppmv higher than when the NaClO 2 concentration was 1.00 mmol/L. NO 2 was transported by the escape of ClO 2 , resulting in a higher concentration of NO 2 [35]. Therefore, the optimal NaClO 2 concentration was considered as 1.00 mmol/L in this study.

Conclusions
Detailed experiments were carried out to study the influence of various parameters on NO removal efficiency, including the ∆P of the HC reactor, the initial pH, the reaction temperature, and the concentration of NaClO 2 . The experimental results showed that removing NO from ship exhaust based on HC using NaClO 2 solution was a valid method. The advantages of this novel method were low NaClO 2 concentration and high NO x removal efficiency. The NO x removal efficiency reached 100% for the NaClO 2 concentration of 0.60 mmol/L. The HC reactor could generate many gas-filled bubbles with small volumes, which was conducive to enhancing the contact area between liquid and gas to accelerate the reaction rate. The reduction of initial pH could significantly improve the oxidation capacity of the NaClO 2 solution. η NOx initial was below 100% for the initial pH = 4.00-7.00. When the initial pH ≤ 3.50, η NOx initial reached 100% and was maintained for more than 8 min. The fundamentals for NO x removal by NaClO 2 solution under acidic conditions was the rapid activation of ClO 2 , and the increasing Tη NOx,100% required improved NO 2 absorption. Additionally, • OH and • ClO produced by HC promoted the NO 2 absorption, which may be one of the reasons for complete NO 2 removal when η NOx was 100%.